Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which can be used in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB such as (Cu), silver (Ag), gold (Au), Group IIIA such as boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl), and Group VIA such as oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax(SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications. Among these compounds, Cu (In,Ga) (S,Se)2 is the most advanced and solar cells in the 12-20% efficiency range have been demonstrated using this material as the absorber. Aluminum containing chalcopyrites such as Cu(In,Al)Se2 layers have also yielded over 12% efficient solar cells.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which includes a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer 13 or contact layer, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. The substrate 11 and the conductive layer 13 form a base 13A (not shown) on which the absorber film 12 is formed. Various conductive layers comprising molybdenum (Mo), tantalum (Ta), tungsten (W), titanium (Ti), stainless steel and the like have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use the conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a cadmium sulfide (CdS), zinc oxide (ZnO) or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1.
In a thin film solar cell employing a Group IBIIIAVIA compound absorber, the cell efficiency is a strong function of the molar ratio of IB/IIIA. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the properties. For a Cu(In,Ga)(S,Se)2 absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In+Ga). Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor vary with the molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for good device performance Cu/(In+Ga) molar ratio is kept at around or below 1.0. As the Ga/(Ga+In) molar ratio increases, on the other hand, the optical bandgap of the absorber layer increases and therefore the open circuit voltage of the solar cell increases while the short circuit current typically may decrease. It is important for a thin film deposition process to have the capability of controlling both the molar ratio of IB/IIIA, and the molar ratios of the Group IIIA components in the composition. It should be noted that although the chemical formula is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k value is 2, although it is typically close to 2 but may not be exactly 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
If there is more than one Group VIA material or element in the compound, the electronic and optical properties of the Group IBIIIAVIA compound are also a function of the relative amounts of the Group VIA elements. For Cu(In,Ga)(S,Se)2, for example, compound properties, such as resistivity, optical bandgap, minority carrier lifetime, mobility etc., depend on the Se/(S+Se) ratio as well as the previously mentioned Cu/(In+Ga) and Ga/(Ga+In) molar ratios. Consequently, solar-to-electricity conversion efficiency of a CIGS(S)-based solar cell depends on the distribution profiles of Cu, In, Ga, Se and S through the thickness of the CIGS(S) absorber.
In the fabrication of CIGS films, various manufacturing techniques involving evaporation, sputtering or electrodeposition are used. Evaporation is an expensive technique and non-uniformity problems over large surface areas preset difficulties. Although the sputtering can be applies over large surface areas, it requires expensive systems and sputtering targets. Electrodeposition offers a low-cost alternative for depositing CIGS precursor films in a high-volume manufacturing environment. Electrodeposition is a versatile deposition method with ability to yield thin films of metals, metal alloys and compounds which may be used in a wide variety of precursor layer structures. Electrodeposition equipment is low cost and the process is energy efficient since it is typically carried out at low temperatures. Materials utilization in electrodeposition processes can be close to 100% if stable electrolytes with long lifetime are employed. Electrodeposition is also suitable for high throughput roll to roll manufacturing.
One prior art approach to form CIS and CIGS precursors by electrodeposition is to form stacks consisted of individual elemental layers. Precursor stacks such as Cu/In, Cu/In/Se, Cu/In/Ga, and Cu/In/Ga/Se stacks can be electrodeposited on Mo coated substrates to form Mo/CIS and Mo/CIGS structures and subsequently annealed in inert or Se containing environments to manufacture CIS and CIGS absorber layers. For example, U.S. Pat. No. 4,581,108 describes a low cost electrodeposition method to prepare a metallic precursor preparation. In this method a Cu/In stack is first formed by electrodeposition on a substrate and the stack is heated in a reactive atmosphere containing Se to form a CIS absorber layer. Similarly, Fritz et al. used electrodeposition to form a Cu/In/Se stack on a substrate and a following rapid thermal annealing of the stack to form CIS [Fritz et al., Thin Solid Films 247 (1994) 129]. In another prior art approach developed at SoloPower Inc., a Cu/In/Ga/Se precursor stack is first electrodeposited and converted to CIGS absorber by a subsequent rapid thermal processing step [Basol et al., Proc. 23rd European PVSEC, 2008, p. 2137]. One of the reasons for selecting Cu/In and Cu/In/Ga electrodeposition sequence is the fact that Cu, In and Ga can have very different standard plating potentials. The standard electrode potentials of Cu/Cu2+, In/In3+ and Ga/Ga3+ metal/ion couples in aqueous solutions are about +0.337 V, −0.342 V, and −0.52 V, respectively. This means that Cu can be plated out at low negative voltages. For In deposition, on the other hand, larger negative voltages are needed. For Ga deposition, which is challenging due to hydrogen evolution, even larger negative voltages are required. Therefore, to form a stack including Cu, In and Ga, Cu was typically electroplated first. This was then followed by deposition of In and then Ga so that while plating the second metal over the first metal, the first metal does not dissolve into the electrolyte of the second metal. Therefore, prior-art methods have employed Cu/In/Ga stacks electroplated in that order, which limits the way in which Cu, In and Ga is distributed through the thickness of the precursor film.
One step electrodeposition of CIS or CIGS precursor films from a single electrolyte is another prior art approach for utilizing electrodeposition for CIGS cell fabrication as described in U.S. Pat. No. 7,297,868. The precursor films plated from Cu—In—Ga—Se electroplating bath are subsequently subjected to a high temperature crystallization step to improve their photovoltaic properties. In this prior art, an acidic electrolyte with a pH of approximately 2 was used. The deposition bath used for the codeposition of Cu—In—Ga—Se by electrodeposition contained 0.02M Cu(NO3)2.6H2O, 0.08M InCl3, 0.024M H2SeO3, and 0.08M Ga(NO3)3 and 0.7M LiCl dissolved in de-ionized water. Similar acidic electrolytes for the co-deposition of CIS and CIGS precursors have been investigated by several other researchers. For example, Babu et al. electrodeposit CuInSe2 from a sulphate bath containing 10 mM CuSO4, 50 mM In2(SO4)3 and 30 mM SeO2 with a pH of 1.5 [Babu et al, Journal of Crystal Growth 275 (2005) e1241-e1246]. In another example, Sene et al. employ sulfate-based plating baths, containing CuSO4.5H2O, In2(SO4)3.H2O, SeO2 and Li2SO4.H2O as a supporting electrolyte, dissolved in deionized water, prepared with and without pHydrion pH=3 buffer to deposit the CIS films. The pHydrion pH=3 is a mixture of sulfamic acid and potassium biphthalate. [Sene et al, Thin Solid Films 516 (2008) 2188-2194].
In such one-step cathodic electrodeposition processes, simultaneous reduction of all the constituent ions of Cu, In, Ga and Se at the same potential in suitable proportions is necessary in order to achieve the desired film composition. A common problem associated with electrodeposition from such single electrolytes is precipitations of metal oxide and hydroxide. In order to prevent this unwanted deposition, pH buffer and complexing agents are included in the electrolytes. However, even with this approach it is highly difficult to attenuate precipitation and deposition of hydroxides during film growth from acidic solutions. Metal concentrations in the electrolytes are kept in minimum to avoid this problem. While this can be an acceptable approach for studying the fundamentals of electroplating precursors in research scale, industrial large plating applications require plating baths with stable compositions which can be kept for several months.
Another major problem in the co-electrodeposition of Cu—In—Ga—Se from acidic electrolytes is generation of colloidal Se which is mostly produced near the cathode surface. These colloidal Se particles aggregate and become larger in size with time. As the plating continues, both the number and the size of red selenium particles increase in the electroplating solution. Some particles get trapped on the cathode surface and form defects in the deposited Se film in the form of particle inclusions. Such compositional differences between portions of the stack create morphological, electrical and compositional differences between corresponding portions of the compound CIGS layer obtained after the reaction step. This, in turn, reduces the CIGS layer's uniformity and thus reduce the efficiencies of solar cells fabricated on such non-uniform layers. One approach to minimize this problem is to use very slow deposition rates in the co-electrodeposition of CIGS. Deposition periods exceeding 45 minutes are not uncommon. For example, Sene et al. indicate that it took 90 minutes to obtain a approximately 2 μm thick CIS layer [Sene et al, Thin Solid Films 516 (2008) 2188-2194]. Obviously, such extremely low deposition rates are not appropriate for large scale manufacturing purposes such as roll to roll manufacturing.
Some of the problems described above can be avoided if selenium is excluded from the single step electrodeposition solution. A Cu—In—Ga electrolyte can be used to deposit only a ternary thin film layer of Cu—In—Ga as described by Ganchev et al., Thin Solid Films 511-512 (2006) 325-327. This Cu—In—Ga bath contained 50-100 mg cuprous chloride (CuCl), 100-350 mg indium chloride (InCl3), 1700 mg gallium nitrate (Ga(NO3)3.7H2O) and 2M potassium thiocyanate (KSCN) as a complex agent in 0.2 liter of de-ionized water. The pH=5 value of the solution was adjusted by 0.4 M acetate buffer. The publication indicates thiocyanate is used to shift the deposition potential of elemental copper to bring it closer to deposition potentials of Ga and In. It is noted that thiocyanate complexes of In3+ and Ga3+ are not as stable as cuprous complexes. As a result, In3+ and Ga3+ reduction potentials did not change noticeably. Since thiocyanate cannot form stable complexes with In and Ga ions at this pH, formation of indium and gallium oxides and hydroxides cannot be completely eliminated. Due probably to the instability issues, this prior art formulation for Cu—In—Ga electrolyte was extremely sensitive to the hydrodynamic regime of deposition as evident from the large changes in the Ga content with and without stirring during plating.
From the foregoing there is a need for better electrodeposition techniques to form various metallic precursor stacks comprising Cu, In, Ga and Se together.